1. Field of the Invention
The present invention relates generally to sealing structures in spindle motors, and in particular, sealing structures having improved geometries for reducing lubricant leakage from bearing assemblies.
2. Description of Related Art
As the hard disk drive industry demands higher performance spindle motors, the bearing assemblies are required to rotate at faster speeds. As the bearings rotate, submicron sized particles from the bearing lubricant often escape from the bearing assemblies into the disk environment. Within the disk environment, one or more disks, and the air around the disks, are rotating within a sealed enclosure. The aerosoling lubricant that escapes from the spindle motor often swirls around with the air in the disk environment and eventually deposits itself on the surface of the disks and/or on the air bearing slider which supports the read/write transducer or "head."
When the disk drive is turned off, the air bearing generated between the rotating disks and the sliders are no longer generated, and the sliders generally come into contact with the disk surface. If one or more sliders "land" in an area that is covered by the bearing lubricant, the sliders will have a tendency to stick to the disk surface during start-up of the disk drive. In order to overcome this sticking force during start-up, a high motor current (to produce higher torque) is necessary, often resulting in damage to the suspension that supports the slider. In severe cases, the disk drive fails to start altogether. Thus, the contamination caused by the bearing lubricant at the head-to-disk interface degrades the performance of the disk drive.
Spindle motors used in conventional hard disk drives often use a sealing structure to minimize the lubricant leakage from one or more bearing assemblies, particularly the lower bearing assembly. Conventional sealing structures include one or more labyrinth seals, which are formed of a sealing surface on one component and a restrictive ring on an opposing component such that the ring projects into close proximity to the sealing surface. Typically, one of the components is a stationary part of the spindle motor and the other component is a rotating part of the spindle motor. The term "seal" generally refers to a tight fitting or closure with some clearance or space between two surfaces. Thus, a sealing structure formed by one or more labyrinth seals provides an intricate path that inhibits, if not prevents, the outward movement of contaminant particles from the spindle motor. Such sealing structures make it possible to considerably reduce the risk of lubricant particles from passing from the inside of the spindle motor into the disk environment.
FIG. 1A illustrates a conventional sealing structure having a single labyrinth seal used in a spindle motor. The labyrinth seal shown in FIG. 1A is used to minimize the lubricant leakage from a lower bearing assembly 110 and is formed between the upper surface of a motor mounting flange 120 and the lower surface of a bearing bushing 130. The bearing bushing 130 supports a bearing assembly 110. The bearing assembly 110 is typically a ball bearing assembly having a stationary portion and a rotating portion. A gap is formed between the rotating and stationary portions of bearing assembly 110. During operation, mounting flange 120 remains stationary while bearing bushing 130 rotates.
When lower bearing assembly 110 is operating at high speeds (i.e., 7200 rpm, 10,000 rpm, etc.), the centrifugal force of the spindle motor causes submicron airborne particles to escape through the gap formed between the rotating and stationary portions of bearing assembly 110. The submicron particles come from the bearing lubricant which is typically a mixture of oil and grease and are often in the form of bubbles.
The centrifugal force causes the particles to escape from bearing assembly 110 through the gap and to move into a clearance 121 between the upper surface of the mounting flange 120, which is stationary, and the lower surface of the bearing bushing 130, which is rotating. Air viscosity and air friction cause the thin layer of air and the air and grease bubbles contained therein, to rotate. This rotation subjects the oil and grease bubbles to a centrifugal force which drives them from the inner diameter toward the outer diameter of the spindle motor as shown by the arrows in clearance 121.
The mounting flange 120 includes a single annular protrusion 125 that extends into a single annular groove formed in bearing bushing 130. The annular protrusion 125 and annular groove 126 are separated by a gap to form the labyrinth seal. Although the sealing structure formed by the single labyrinth seal shown in FIG. 1A is effective enough to prevent some of the bearing lubricant particles from leaking into the disk environment, the centrifugal force is often large enough to force some of the particles out of the labyrinth seal.
Other conventional disk drives use a sealing structure that includes multiple labyrinth seals. For example, Japanese Publication 53-76809, discloses a sealing structure having multiple labyrinth seals 170-172 as shown in FIG. 1B. The multiple labyrinth seal structure 160 increases the constriction effect of a single labyrinth seal, due to the fact that with each passage through a single labyrinth structure, a significant amount of the dynamic pressure of the leakage flow is lost due to flow around sharp edges, thereby maximizing the pressure difference between an inlet 161 and an outlet 162 of the sealing structure 160. As the protruding annular rings are positioned closer towards outlet 162 of the labyrinth seal, the height of the protruding annular rings are shorter. Each annular ring also includes a sharp surface at its outer circumference to increase the dynamic pressure loss of the leakage flow. During the manufacturing of the labyrinth seal, each sharp surface is machined individually, which is time consuming. Although this labyrinth seal may be somewhat effective in preventing lubricant particles from leaking into the disk environment, it is relatively cumbersome to manufacture and therefore not practical from a cost standpoint.